Whipstocks are well known in the mining and petroleum industries and are used to change the direction of a drill hole (directional drilling). Since the earliest times, boreholes were made to deviate by placing tapered wedges or “whipstocks” in the borehole to force the bit sideways into a new direction, and it is well known that different bottom-hole assemblies had a tendency either to increase or decrease the inclination of the hole. No one drilling method is satisfactory for all radii of curvature. It is therefore customary to distinguish among these as long-, medium-, short-, and ultra-short-radius methods. The invention relates to ultra-short-radius methods which are typically defined to have a radius of 2 ft. (0.6 m) or less.
Directionally drilled wells fall into two main categories. In the first category, the task is to reach locations that are not accessible through straight, vertical holes. The objective is to reach a substantial distance horizontally away from the drilling location. The second category consists of wells in which part of the well that lies in a particular oil or gas reservoir is given a particular orientation so as to increase productivity. An example of this second category is a vertically thin reservoir where a horizontal hole can contact a greater part of the reservoir than a vertical one, increasing the drainage contact area. It is this second category to which the invention primarily relates.
Ultra-short-radius whipstocks have been developed that are applicable to the second category of directional drilling.
A common feature of existing ultra-short-radius whipstocks is a requirement to incorporate a device in the bottom-hole assembly that either pushes the drill string around the ultra-short-radius and into the deposit which is to be drilled, or uses a complicated hydraulic piston drive arrangement. Drill strings for use with these whipstocks are either segmented or coiled tubing.
More elaborate devices are also known to turn a drill string and/or cutter into the borehole wall. For instance, U.S. Pat. No. 5,197,783 describes a cavity forming device for use in boreholes and which has an erectable arm member provided with a fluid cutting jet to cut a large cavity in the borehole.
U.S. Pat. No. 4,497,381 describes a drill string bending assembly which has an extending arm portion to direct the drill string into the sidewall.
A disadvantage with existing whipstocks and other similar assemblies is in correctly determining various parameters in the down hole and drilling process. For instance it is necessary to determine the distance travelled by the fluid cutter, whether the segmented or coiled tubing drill string is feeding properly through the bore, when the fluid cutter is properly retracted so that the arm member can be retracted, the orientation of the arm member, the degree of inclination of the arm member, and so on.
The assembly of the invention can be used with a drilling system that uses a high pressure hose as a flexible hose drill string and a self-advancing fluid jet cutting nozzle. Such a nozzle has been described in International Application No. PCT/AU96/00783.